Won't start with good 12V battery

Can you post a photo? You're not talking about something in the long white fusible link assembly?

 

Oh now you're onto something. Maybe someone jumped started this car, accidentally reverse polarity the battery connection (if you did it in the rear, it could happen because of the tight space). That will blow the fusible link and the car won't start

 

If there was a reverse polarity jump performed that opened the fusible link in the white plastic housing (with clear plastic top) I saw a way to work around it on this site if the car needs to be moved. The post mentioned it should definitely only be temporary, as it basically used pennies wedged on each side of the blown link to get continuity back. A bit risky, but sometimes that's required if you need to get a car moved. If you don't know what caused it to blow, I don't think I would do that penny trick. I'll try to find that post, it's been months since I read it.

Found it, it even has a photo.

2008 Prius won’t start | PriusChat

Don't forget, when a reverse polarity jump occurs, it normally blows a couple fuses in that fuse box also.

Good luck!